Chatoyant Gems - The Cat’s-Eye Phenomenon
What causes the bright line of reflected light that moves under the surface of a chatoyant gem?
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
What is Chatoyance?
Chatoyance is an optical phenomenon in which a band of reflected light, known as a "cat's-eye," moves just beneath the surface of a cabochon-cut gemstone. Chrysoberyl and tiger's-eye are two of the best-known gem materials that exhibit this phenomenon. Excellent specimens of chrysoberyl exhibit the finest chatoyance, and tiger's-eye is the chatoyant gem most widely used in jewelry.
Chatoyance occurs in stones that contain a large number of very thin parallel inclusions within the stone, known as a "silk." The light reflects from these inclusions to form a thin band across the surface of the stone. The band of light always occurs at right angles to the length of the parallel inclusions. These inclusions can be crystals, hollow tubes, or other linear structures that are present throughout the stone and are usually aligned with a crystallographic axis. Needle-like crystals of rutile and hematite are well-known for producing a cat’s-eye in many specimens.
Chatoyance - Eye of the Cat
The name "chatoyance" originates from the French word "chatoyer," which means "to shine like a cat's eye." The analogy also matches the way a cat's pupils will narrow to a thin slit under bright light.
A good analogy for the chatoyance effect is how light reflects to form a line across the surface of a spool of silk thread. Viewing and moving a spool of silk thread under a beam of incident light can be a useful way to develop an understanding of how parallel inclusions produce an "eye" within a gemstone.
In a chatoyant gemstone, the band of light will move back and forth beneath the surface of the gem as it is turned under a beam of incident light. The band will also move if the position of the light is moved, or the observer moves his head to view the stone from a different angle. The motion of the cat’s-eye across the top of the gem is one of the things that makes these stones so interesting, beautiful, and desired by many people.
Many Types of Gems Exhibit Chatoyance
Many types of gems exhibit chatoyance; however, it is not present in every specimen of those gems and is usually only seen in a minority of specimens. Gems with well-known chatoyance include the following: chrysoberyl, tiger’s-eye, actinolite, apatite, beryl (aquamarine, heliodor, emerald), beryllonite, cerussite, danburite, diaspore, diopside, enstatite, garnet, iolite, kyanite, moonstone, opal, peridot, pezzottaite, prehnite, quartz, rutile, sillimanite, scapolite, spinel, topaz, tourmaline, and zircon.
Milk and Honey
Some gems with a highly developed chatoyance can appear to be made of two different materials when illuminated from the proper direction with respect to the observer's eye. In these stones, the cat’s-eye will appear to divide the stone into a zone of light-colored material on one side of the eye and dark-colored material on the other. This phenomenon is known as the "milk-and-honey" effect. A photo of cat's-eye chrysoberyl showing the milk-and-honey effect is included below.
Cutting the Cat's-Eye
Anyone who cuts cabochons with the intent of producing a cat's-eye stone must first find a material with a silk of inclusions that is capable of producing an eye. Then the cutter must examine the rough and orient the stone so that the silk will be parallel to the bottom of the stone within the finished cabochon, and the parallel inclusions will be parallel to the long axis of the cabochon. If this proper orientation is not achieved, the stone will have an off-center eye or no eye at all.
Assessing the Quality of a Cat's-Eye Gem
The best cat's-eye gems have an eye that meets the following criteria:
- it is clearly visible
- it symmetrically bisects the cabochon
- it contrasts sharply with the stone's body color
- it moves smoothly as the stone is turned under light
Diffraction in Cat's-Eye
Rare chatoyant specimens will have a coarse silk with just the right spacing to serve as a diffraction grating. These specimens will produce not only a cat's-eye, but also a display of spectral colors caused by light passing through the coarse silk and being diffracted into the colors of the spectrum. More familiar examples of diffraction are rainbows produced by sunlight diffracted by raindrops, or opal's play-of-color caused by light diffracted by tiny spheres of opal within the gem. The specimen of cat's-eye scapolite in the photo on this page is an example. (Another stone that exhibits diffraction is iris agate.)
Purchasing Cat's-Eye Gems
If you like cat's-eye gems and want to shop for them, they are very easy to find on websites and in stores that specialize in loose gems. The customers who shop at these stores are usually jewelry designers and gemstone collectors.
Cat's-eye gems are rarely seen in the mass-production jewelry typically sold in department stores and shopping mall jewelry stores. Why? Because every cat's-eye stone is cut from a piece of rough that must be studied and oriented properly before cutting. After that work, the cutter almost always tries to fashion a stone that will produce the largest gem possible. As a result, cat's-eye gems are rarely produced in the calibrated sizes needed for mass-production jewelry shops.
The only cat's-eye gem that is commonly seen in mass-production jewelry is tiger's-eye. It is popular in men's rings and cufflinks and is also very popular in beads. Chrysoberyl is occasionally seen in jewelry stores, but it is very expensive in sizes that are large enough to be used in a ring.
If you are looking for a piece of jewelry with a nice cat's-eye gem, the best place to shop is at the website or store of a jewelry designer. They are the artists who most appreciate these phenomenal gems and enjoy creating a special piece of jewelry to display a unique gem. Some jewelry designers have an inventory of cat's-eye gems or will help you shop for a gem that suits your color preference, size, and price range. They will then create a special piece of jewelry just for you.
Asterism is a type of chatoyancy in which the viewer sees the intersection of multiple cat's-eyes, or a "star," rather than a single cat's-eye. Tightly packed, parallel inclusions that are oriented in two different directions will cause a star with four rays. If there are three sets of inclusions, the viewer will see a star with six rays.
The most well-known examples of asterism in gemstones are star ruby and star sapphire. The accompanying photo shows a black sapphire with a six-rayed star. Stars can also be found in other gems such as garnet, spinel, diopside, and quartz.
|Ruby and Sapphire|
|Pictures of Opal|
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